The existing long-distance logistics transportation lines generally apply low-cost linear induction motors, whose power factor and efficiency are low. The PM structure can improve performance. Since the traditional PM synchronous motors generally use short armature movement, the magnetic pole length is decided by the distance. The application field is extremely limited by the amount of PM and the high cost for long stroke applications. For conventional LVPM motor, the armature windings are located on the secondary and the PMs are located on the primary. It also has the same problems with the traditional PM synchronous motor that the invest cost will be huge in the long stroke applications.
On the other hand, with the deepen studies on the stator PM motors and VPM motors, more attentions are paid to flux-switching, flux-reversal and doubly-salient PM motors and the VPM motors based on magnetic field modulation effect. In these motors, the VPM motor can modulate the low-speed magnetic field into high-speed magnetic field, which has the characteristic of high torque at low speed. However, it has the problems of much magnetic flux leakage, the low utilization of PM and low power factor.
Chinese Invention Patent No. 201010119957.X discloses a long stator primary linear PM motor. In fact, it is a doubly-salient PM linear motor. The PM field is unipolar and PMs and windings are placed on the primary side. It can save cost for long stroke applications. The motor models of the specific embodiments 3 and 4 are much closer to traditional doubly-salient PM motor. However, it does not have the ability to modulate the magnetic field. There are more useless harmonic content in the air gap flux density, which limits the output thrust force. Meanwhile, the losses generated by these useless harmonics exacerbate the fever of the PMs, which go against to the heat dissipation of primary side. Chinese Invention Patent Application No. 201510209749.1 discloses a stator PM motor. Its structure is similar to doubly-salient PM motor. But from the magnetic field distribution and operation principle shown in FIG. 6, it is closer to flux-switching motor, not conventional doubly-salient motor. Although the motor creates a bipolar armature PM flux and smooth torque output, the motor places a PM every two stator salient teeth, and at the same time the flux of each winding turns is generated only by a piece of PM. The stator salient teeth do not have the ability to modulate magnetic field, which limits the magnitude of the back-EMF and output torque. Compared to the unilateral linear motor, the thrust force density of bilateral linear motor can be further improved, but there are not many studies on the DS linear PM motor. Chinese Invention Patent Application No. 201110000290.6 discloses a bilateral flux-switching linear PM motor, which can produce a stable thrust force. However, similar to ordinary flux-switching PM motor, it does not have the ability to modulate the magnetic field. Although the end magnetic flux leakage is limited compared to traditional unilateral flux-switching motor, the bilateral magnetic fields are not connected in series. Therefore, the effect is just simply superimposed by two unilateral motor, or even less than double times of a unilateral motor. Chinese invention Patent Application No. 201410259315.8 discloses a DS-LVPM motor. The two sides of the stator are only made of silicon steel sheet. Windings and PMs are located on the primary side, but PM is also fixed on the surface of the primary teeth, which cannot avoid serious magnetic flux leakage at the ends of the PMs, resulting in the low power factor and limiting the output thrust force. Its bilateral stators are perfectly symmetrical, which cause the magnetic circuit on both sides cannot be connected in series, so the effect is simply superimposed by unilateral motor. It cannot give full play of the characteristics of the magnetic field modulation effect.